Amplifying pusles of different duty cycles

ABSTRACT

An amplifier stage capable of delivering a peak limited voltage pulse with sharp transitions, at a desired width and duty cycle, and with high efficiency is disclosed. One disclosed embodiment relates to a circuit that includes a tuned class D amplifier that receives an input signal and generates a pulsed RF output signal in response to the input signal. The pulsed RF output signal has a greater power than that of the input signal.

RELATED APPLICATIONS

This application claims priority under 35 U.S.C. § 119(e) to U.S.Provisional Application Ser. No. 60/866,147, entitled “Electronic SwitchNetwork,” filed on Nov. 16, 2006; U.S. Provisional Application Ser. No.60/866,144, entitled “Distributed Multi-Stage Amplifier,” filed on Nov.16, 2006; and U.S. Provisional Application Ser. No. 60/866,139, entitled“Pulse Amplifier,” filed on Nov. 16, 2006. Each of the foregoingapplications is hereby incorporated by reference herein in its entirety.

BACKGROUND OF INVENTION

1. Field of Invention

This invention generally pertains to methods and devices for amplifyinga signal. In certain aspects, it relates to power amplifiers andamplifier systems working at radio frequencies (RF) or higher.Applications include, but are not limited to, wireless systems,microwave components, power amplifiers, CMOS amplifiers, driveramplifiers, and portable electronics.

2. Discussion of Related Art

A common problem in power amplifier (PA) design is dealing with thebreakdown limitations of the device technology. Most techniques forpower amplification produce a peak voltage on the terminals of thedevice between two and four times the supply voltage. It is oftendesirable to tune the amplifier to create a peak voltage as high aspossible to improve the efficiency of the amplifier. However, this peakvoltage must stay well below the breakdown limits of the devicetechnology. This creates a problem for technologies such as CMOS whichhave very low breakdown voltages. For example, in a wireless handset thesupply voltage can be nominally 3.5V and the peak voltage for anefficient amplifier can be at least 7.0V. A 0.5 um CMOS processtypically only has a breakdown voltage of 5.0V, making the technologyunsuitable for the application.

A second problem can arise when a power amplifier is used to drive anantenna or other uncontrolled load impedance. In the case of an antenna,the PA might see a load impedance that varies by a factor of as much asten. This can cause the PA to deviate from its nominal class ofoperation and produce peak voltages significantly higher than planned.For this reason, it may be desirable to use a device technology with abreakdown voltage of greater than four to five times the supply voltage.

Several techniques have been employed in the industry to avoid theseproblems. Multiple cascade stages have been used to reduce the voltageacross any one transistor. Amplifiers can also be implemented in serieswith the supply to divide the voltage swing across two or more sets oftransistors. Either of these techniques can solve the first problem butwill struggle with the second issue of load tolerance. A DC-DC convertercan also be used to control the supply voltage. However, this will havea significant impact on the cost of the system and may also strugglewith the second problem.

Push-pull class D amplifiers have the advantage of keeping the voltageat or below the supply voltage for all conditions. While this solves thetwo major problems discussed, they have poor DC to RF conversionefficiency at RF frequencies. This is because the output capacitance ofthe two devices must be discharged each time the amplifier switchesstate. The resulting power loss is 2·π·F·C_(out)·(V_(sw))², where F isthe switching frequency, C_(out) is the output capacitance and V_(sw) isthe voltage across a switch upon switching. This power loss isproportional to the switching frequency, F, and is unacceptably high atRF for most commercially available device technologies.

One variant of this technique that retains the advantage of low peakvoltages while producing high efficiency is the class DE amplifier. Thiswas first suggested by Zhukov and Kozyrev in 1975. Its most widespreaduse has been for rectifiers for DC converters. The basic idea is toimprove the efficiency of a class D push-pull amplifier by controllingthe switching duty of the two devices. Typically, the biggest source ofpower loss in a push-pull amplifier at RF is the energy that isdissipated while charging the output capacitance of the devices duringtransitions. FIG. 1 a shows a push-pull amplifier with the devices drawnas ideal switches with parasitic output capacitance. As the bottomswitch transitions from the off state to the on state, it must dischargethe full supply voltage present at its drain. Since these losses can beincurred in each cycle, the overall power dissipation caused by thebottom switch is 0.5·ω·C_(n)·(Vsup)² (where ω=2·π·F, C_(n) is theparasitic output capacitance of the bottom switch, and Vsup is thesupply voltage). A similar loss of 0.5·ω·C_(p)·(Vsup)² is incurredduring the transition of the top switch. The total power loss incurredby both switches is thus represented by ω·(C_(n)+C_(p))V²=ω·C_(out)·(Vsup)². At RF frequencies this can be a significant lossin power and efficiency. The class DE amplifier can overcome thisproblem by switching the devices with independent signals and creating aperiod of time where both transistors are simultaneously off. A tunedoutput network can be used to provide the current necessary to dischargethe output capacitance before the switch turns on. A diagram of thisoperation is shown in FIG. 1 b. The class E condition of zero-voltagewith zero-slope switching can be imposed to realize high efficiency.Efficiencies can be obtained that rival or surpass the techniquespreviously discussed while maintaining a peak voltage that is no greaterthan the supply voltage. Moreover, this circuit can be designed suchthat the peak voltage will never be higher than the supply, even underVSWR (Voltage Standing Wave Ratio) mismatch conditions. The penaltiesfor this advantage can include a lower power density (a by-product ofthe lower peak voltage) and a more complex input drive. The low gain ofmost devices at RF and the complexity of the system have made realizinga class DE amplifier impractical at RF frequencies.

SUMMARY OF INVENTION

Exemplary methods and circuits capable of delivering a peak limitedvoltage pulse with sharp transitions, at any width and duty cycle, andwith high efficiency are disclosed. At a duty cycle of 50%, for example,the output voltage waveform may resemble a square wave. Such a circuitis suitable as a driver stage for many different types of RF amplifiers,and in one embodiment it is used as a driver stage for a class DEamplifier.

In accordance with certain embodiments of the present invention, such acircuit may be designed to operate at RF frequencies with high DC to RFconversion efficiencies. Such a circuit can be realized using, but notlimited to, the following technologies: silicon bipolar transistors,CMOS transistors, GaAs MESFETs, GaAs HBTs, GaAs PHEMTs. Such a circuitcan also be compatible with the various IC manufacturing processesassociated with the above technologies and can yield a monolithicsolution.

According to one exemplary embodiment, a circuit can include a push-pullamplifier having a tuned load network connected to its output. Thepush-pull amplifier can have one or more switching devices. The tunedload network can be connected in parallel across the output, and can beconfigured such that the switching devices operate under substantiallyzero-voltage and/or zero-slope switching conditions. It also can beconfigured such that the output of the push-pull amplifier is notfiltered and/or retains higher order harmonics, including harmonicssufficient to generate a trapezoidal, square wave, or any othernon-sinusoidal waveform.

Another embodiment relates to a circuit that includes a tuned class Damplifier that receives an input signal and generates a pulsed RF outputsignal in response to the input signal. The pulsed RF output signal hasa greater power than that of the input signal.

Yet another embodiment relates to a circuit for amplifying a signal. Thecircuit includes a class D amplifier and an input circuit. The inputcircuit provides pulsed input signals to drive the class D amplifier. Atleast two of the pulsed input signals have different duty cycles.

A further embodiment relates to a method of operating a class Dpush-pull amplifier to generate a pulsed output signal. The class Dpush-pull amplifier includes a push transistor and a pull transistor.The push transistor is driven with a first input signal having a firstduty cycle. The pull transistor is driven with a second input signalhaving a second duty cycle. The first and second duty cycles of thefirst and second input signals are controlled such that the pulsedoutput signal has a desired duty cycle.

BRIEF DESCRIPTION OF DRAWINGS

In the drawings, each identical or nearly identical component that isillustrated in various figures is represented by a like numeral. Thesedrawings are not necessarily drawn to scale. For purposes of clarity,not every component may be labeled in every drawing. In the drawings:

FIG. 1 a shows a push-pull class D amplifier power stage;

FIG. 1 b shows a common amplifier stage;

FIG. 1 c shows another common RF amplifier stage;

FIG. 2 shows the operation of a class DE stage having a filtered output;

FIG. 3 shows a class DE amplifier stage that is driven using asinusoidal input signal provided by a transformer;

FIG. 4 shows a circuit capable of creating pulsed drive waveforms inaccordance with an exemplary embodiment of the invention;

FIG. 5 shows the circuit of FIG. 4 operated so as to produce an outputwaveform having a duty cycle of less than 50%, according to oneembodiment;

FIG. 6 shows the circuit of FIG. 4 operated so as to produce an outputwaveform having a duty cycle of greater than 50%, according to anotherembodiment;

FIG. 7 shows exemplary timing diagrams for input producing signals withvarying duty cycles that may be used to drive a pulse amplifier,according to one embodiment;

FIG. 8 shows how two pulse amplifier circuits can be used to drive aclass DE output stage, according to some embodiments; and

FIG. 9 shows the circuit of FIG. 8 configured to functiondifferentially, according to some embodiments.

DETAILED DESCRIPTION

FIG. 1 c shows the configuration and associated waveforms for a commonRF amplifier stage. An applied input signal is passed through a tunedinput matching network 102 that filters the applied input signal andadjusts the relative impedances so as to optimize gain. Transistor 101amplifies the input waveform to create a larger output signal. Theamplifier 100 may be designed to operate in the linear region or in thesaturated region so as to produce high DC to RF conversion efficiency.In the latter case, the voltage waveform 108 at node Vx will take onvarious characteristics according to the mode of operation used for thedesign. For class F amplifiers, the voltage waveform 108 at node Vxtypically will approximate a square wave signal with a peak of 2·Vsup.Class E and other tuned modes generally produce peak voltages that arehigher. Push-pull amplifiers will have a peak voltage which issubstantially equal to the supply voltage Vsup. The output signal inpush-pull amplifiers usually has a duty cycle of 50%. The mode ofoperation for the amplifier is determined by the design of elements 103,104, 105, and 106. As shown, bias inductor 103 is used to isolate thesupply voltage Vsup from transistor 101 and enables the peak voltage ofVx to exceed the supply voltage Vsup. Harmonic match 104 is used tomatch the impedances at the harmonics or the frequency of operation.This is principally responsible for setting up the mode of operation forthe amplifier. For example, a class F amplifier might have evenharmonics set to be short circuits, or zero-Ohm impedances, and oddharmonics set to open impedances, or infinite-Ohm impedances. Theimpedance match 105 can transform the system impedance used for bestperformance of the transistor to the load impedance R_(L) 107. The lowpass filter 106 can be used to filter the harmonics of the signal and isgenerally either required by or a result of the class of operation. Inthis type of amplifier, elements 103-106 operate to heavily filteroutput waveform 109 at Vout such that it approaches a sine wave with 50%duty cycle. This type of waveform is undesirable for driving many outputstage amplifiers, including a class DE stage.

Exemplary operation of a class DE stage is depicted in FIG. 2, whichshows circuit 200. As shown, transistors 201 and 202 are configured tooperate similar to a push-pull amplifier. Higher DC to RF conversionefficiency than a standard push-pull class D amplifier is achieved,however, by turning off each stage prior to turning the other stage on.This creates a period of time where both devices are off, allowing thevoltage at the common node to fully discharge before current flows inthe devices. The waveforms 203 and 204 represent the current throughtransistors 201 and 202. At all other times the device is turned off.The waveform 205 shows the voltage at the common terminal Vx. Carefuldesign of the tuned load network 206 will ensure that each transistorturns on with zero current and voltage across its terminals. Eachtransistor can be operated with non-overlapping duty cycles of less than50% and with precise control of the timing to achieve class DEoperation. This may be achieved by driving each transistor withdigitally generated pulse width modulated signals. However, this is notpractical at RF frequencies with the power levels necessary to drive anoutput stage. The class DE amplifier is also usually only practical ifthe efficiency of the driver stage does not degrade the overallamplifier efficiency.

FIG. 3 shows a circuit 300 that illustrates how a class DE amplifier canbe configured to operate from a sine wave drive signal provided by atransformer. Here, a transformer 301 is used to isolate the DC level forthe two transistors of the push-pull stage. The transistors are thenbiased at or below threshold so that the amplifier has less than a 50%duty cycle. This solution may create several problems. In particular,the sub-threshold bias will further reduce the gain of the output stagedevices. The resulting waveforms at the gates of the transistors mayalso exceed certain breakdown requirements for the devices, which willrequire the addition of diodes or other protection circuitry, furtherreducing gain and increasing complexity.

A U.S. patent application filed on even date herewith and entitled“Distributed Multi-Stage Amplifier” describes a circuit and method fordriving a push-pull amplifier that provides efficient class DE operationof the amplifier at or above RF frequencies. As described herein, such acircuit or method may benefit from the capability of creating pulseddrive waveforms with variable duty cycles at high efficiencies.

FIG. 4 shows a circuit 400 capable of creating pulsed drive waveforms inaccordance with an exemplary embodiment of the invention. The circuit iscapable of generating the drive waveforms with sufficient power and withhigh DC to RF conversion efficiency. As shown, the driver stage 401 isconfigured to operate similar to a standard class DE amplifier.Transistors 402 and 403 are connected between ground and Vdd (a constantsupply voltage). Transistors 402 and 403 are configured in a push-pulltopology and are driven such that each transistor is turned off prior tothe other being tuned on. The combination of transistors 402 and 403 maybe referred to as an inverter. FIG. 4 shows the current waveforms 405and 406 for transistors 402 and 403, respectively.

In a standard class DE amplifier, the push-pull transistors typicallydrive a series resonant circuit designed to discharge the outputcapacitance during the time when both transistors are in the off state.The circuit of FIG. 4 may differ from a standard class DE amplifier withrespect to the output matching network 404. In the output matchingnetwork 404, this series combination described above can be replacedwith a shunt network comprising an inductor in series with a blockingcapacitor C_(blk). If the capacitor is large enough, it can function asa voltage supply with a potential equal to the average voltage seen atVout and serve to supply the inductor current. An additional capacitor Cmay be added to provide further design flexibility or reflect any outputcapacitance present in the load. As shown, the output matching network404 also takes into account the load impedance ZL, e.g., for impedancematching purposes. The load impedance ZL can represent any type of loadthat the amplifier circuit can be driving, for example, a transistor, anamplifier stage, an antenna, an output conditioning circuit, etc.Techniques for the selection of a suitable output matching network 404will be discussed below.

A desired duty output duty cycle Ds may obtained by selecting the OFFtime φ according to equation [2] shown below. As illustrated in equation[1], the OFF time φ may be selected sufficiently low such that thetransistors can supply the desired peak output current I_(peak), given aswitching frequency ω=2πF, output capacitance Cout, and supply voltageVdd. As illustrated in equations [3] shown below, OFF time φ may beselected to be high enough such that there is sufficient time for atransition to occur during the OFF time φ such that the transition iscompleted by the time that switching occurs, thereby achieving a highefficiency of the pulse amplifier. Equations [3] can be used todetermine a sufficient OFF time φ, given a an output capacitance Cout,and load resistance R.

$\begin{matrix}{I_{Peak} = \frac{\omega \; C_{out}V_{DD}}{1 - {\cos \; \varphi}}} & \lbrack 1\rbrack \\{{Ds} = \frac{\pi - \varphi}{2\pi}} & \lbrack 2\rbrack \\{{R^{\prime} = \frac{\sin^{2}\varphi}{\pi}},{R = \frac{R^{\prime}}{\omega \; C_{out}}}} & \lbrack 3\rbrack \\{{X^{\prime} = \frac{\varphi - {\sin \; \varphi \; \cos \; \varphi}}{\pi}},{X = \frac{X^{\prime}}{\omega \; C_{out}}}} & \lbrack 4\rbrack\end{matrix}$

The duty cycle Ds and inductance L can be chosen such that the pulseamplifier 400 has a DC to RF conversion efficiency equal to that of astandard class DE amplifier. The inductance L, capacitance C_(blk)and/or C, and output duty cycle Ds can be chosen such that eachtransistor will turn on with zero voltage across its terminals and whilethe change in voltage is at zero slope. As shown in FIG. 4, the outputvoltage waveform 407 present at the output terminal Vout is unfilteredand retains properties close to a square wave. In some circumstances,the output waveform can resemble a slight trapezoid due to the chargingtime for the output capacitance, but can be near ideal for driving aswitch amplifier stage. In practice, the transitions may be faster thanillustrated in FIG. 4, as FIG. 4 is not drawn to scale for the sake ofillustration.

The choice of load network components and the duty cycle of thetransistors depends on how much output capacitance is to be dischargedand how much current can be supplied by the transistors. Once the OFFtime φ has been selected, a suitable reactance X may be selected usingequation [4] shown above. In the output matching network 404 shown inFIG. 4, X is the reactance of the series L-C_(blk) combination(optionally including shunt capacitance C) at the operating frequency.This reactance, a net inductance, can be used to draw current from theoutput capacitance of the transistors to discharge the voltage duringthe time when both transistors are off. In some cases, the optimum valueof inductance should be close to or equal to the valueCout·Vdd²/I_(peak) ², where Cout includes the output capacitance of thetransistors plus any capacitance present in the load network, Vdd is thesupply voltage, and I_(peak) is the peak output current. In the absenceof a complete analytical workup, the performance can be refined throughoptimization. It should be noted that the output matching network 404 isnot limited to the particular arrangement of circuit elements shown, butcan have a wide variety of elements, including inductors, capacitors,and resistors, in series and parallel arrangements, including a widearray of equivalent circuits, as a suitable reactance X may be obtainedin a variety of different ways. The type of output matching networkselected may be suitable to efficiently discharge the parasiticcapacitances of the transistors without filtering the output signal,thereby resulting in a pulsed output voltage waveform 407.

Pulsed output voltage waveform 407 can be useful for driving anamplifier stage that requires a square wave signal with a 50% dutycycle. However, a class DE output stage usually drives signals with dutycycles of greater than or less than 50%. The pulse amplifier 400 canaccomplish this through careful selection of the duty cycles of theinput signals. The duty cycles of the input signals can be selectedusing known amplifier design techniques, and also taking into accountthe equations [1] and [2] above. These equations are based on the energyneeded to discharge the capacitors in terms of the peak current and theduty cycle of each stage. An aspect to consider in selecting the dutycycles is the amount of time, φ, where each transistor is in the OFFstate. If this time φ is selected to be sufficiently large, the loadnetwork will discharge the output capacitance with zero-voltage,zero-voltage-slope and/or zero-current. To change the output dutyvoltage duty cycle Ds, the duty cycle of one of the transistors may bechanged while decreasing the duty cycle of the other transistor by thesame amount, thus holding the OFF time φ constant. For example, the dutycycle of the PMOS transistor may be increased to increase the outputduty cycle Ds. When increasing the duty cycle of the PMOS transistor,the duty cycle of the NMOS transistor may be decreased by the sameamount that the duty cycle of the PMOS transistor is increased, thuskeeping the OFF time φ constant. By keeping the OFF time φ constant atthe selected value, the load network may discharge the outputcapacitance with zero-voltage, zero-voltage-slope and/or zero-currentregardless of the individual duty cycles of the push-pull transistors,which may be changed to produce an output signal with a desired dutycycle.

FIG. 5 illustrates another embodiment of the invention wherein a circuit500 is operated so as to produce an output waveform having a duty cycleof less than 50%. As shown, input signal 505 drives the NMOS “pull”transistor 502 and is chosen to have a duty cycle of greater than 50%,and input signal 506 drives the PMOS “push” transistor 503 and is chosento have a duty cycle of less than 50%. The time when both transistorsare in the OFF state, φ, can remain the same as in a standard class DEamplifier. The components in output load network 504 can be chosen toproperly discharge the output capacitance during the OFF time, φ.Waveforms 507 and 508 show the currents in transistors 502 and 503,respectively. Waveform 509 shows the resulting output voltage havingless than a 50% duty cycle and characterized by sharp transitions.

Sharp transitions include transitions that occur quickly compared to theswitching period Tsw, which is the inverse of the switching frequency.For example, if the voltage at Vout transitions from zero to Vdd, thetransition may occur in a small fraction of switching period, such asless than 5% of Tsw, less than 2% of Tsw, or less than 1% of Tsw.However, the transition time fraction of Tsw is only one metric fordetermining whether a transition is sharp, and it should be appreciatedthat sharp transitions may be characterized by different metrics. Iftransition time is used as such a metric, the transition time may bemeasured in any suitable way, such as the amount of time the signaltakes to transition from 10% to 90% of the change in signal value, forexample. The transitions may be sharp enough that the waveform of pulsedsignals appear to have a trapezoidal shape or the waveform of a squarewave, when viewed on the time scale of about the switching period Tsw.

FIG. 6 illustrates a further embodiment of the invention wherein acircuit 600 is operated so as to produce an output waveform having aduty cycle of greater than 50%. As shown, input signal 605 drives theNMOS “pull” transistor 602 and is chosen to have a duty cycle of lessthan 50%, and input signal 606 drives the PMOS “push” transistor 503 andis chosen to have a duty cycle of greater than 50%. The time when bothtransistors are in the OFF state, φ, can remain the same as in astandard class DE amplifier. The components in output load network 604can be chosen to properly discharge the output capacitance during theOFF time, φ. Waveforms 607 and 608 show the currents in transistors 602and 603, respectively. Waveform 609 shows the resulting output voltagehaving greater than a 50% duty cycle and characterized by sharptransitions.

It should be noted that the output networks 504 and 604 differ fromoutput network 404. In particular, load impedance ZL has been replacedwith a series resistor (RL) and capacitor (CL) to approximate the inputimpedance of a CMOS device. An additional shunt capacitance (not shown)can also be present to represent the parasitic capacitance of thedriving amplifier and/or the load. This more closely represents the casewhen the pulse amplifier is used as a driver stage.

FIG. 7 shows waveforms for various signals of the embodimentsillustrated in FIGS. 4-6. Waveforms are shown for the NMOS transistorgate voltage VGN, the PMOS transistor gate voltage VGP, the draincurrent Id_N through the NMOS transistor, the drain current Id_P throughthe PMOS transistor, and the voltage Vout. Each of the columns in FIG. 7shows waveforms aligned to show the timing of various signals withrespect to one another. Waveforms 701 illustrate various signals ofcircuit 400 as illustrated in FIG. 4, operated so as to produce anoutput duty cycle of 50%. Waveforms 702 illustrate various signals ofcircuit 500 illustrated in FIG. 5, operated so as to produce an outputduty cycle of less than 50%. Waveforms 703 illustrate various signals ofcircuit 600 as illustrated in FIG. 6, operated so as to produce anoutput duty cycle of greater than 50%. Each of circuits 400, 500 and 600has the same OFF time φ. The waveforms shown in FIG. 7 illustrate thatfor a given OFF time φ, increasing the ON time for the PMOS transistorincreases the duty ratio of the output voltage Vout. The OFF time φ canbe held constant by decreasing the ON time for the NMOS transistor bythe same amount that the ON time for the PMOS transistor is increased.Increasing the ON time for the PMOS transistor in this manner willresult in an increase in the duty cycle of the output voltage Vout, andconversely. For example, waveforms 702 show a relatively small ON timefor the PMOS transistor, as illustration by the relatively smallduration of the trough in the VGP waveform, which results in arelatively small duty cycle of the output voltage. Waveforms 701 and 703show larger ON times for the PMOS transistor, resulting in larger dutycycles of the output voltage.

FIG. 8 illustrates an exemplary amplifier circuit 800, and demonstrateshow two pulse amplifiers 802 and 803 can be configured to drive a classDE output stage 801. As shown, pulse amplifier 802 is configured todrive the NMOS “pull” transistor of class DE output stage 801. Inputsignals 1 and 2 can be chosen so that pulse amplifier 802 produces anoutput waveform 804 with a duty cycle of less than 50% and equal to thedesired duty cycle for the desired operation of output stage 801. Asshown in FIG. 8, an output load network 809 is coupled to the output ofclass DE output stage 801 with components chosen to ensure class DEoperation and produce the filtered output waveform 810. Pulse amplifier803 is configured to drive the PMOS “push” transistor of class DE outputstage 801. Input signals 3 and 4 can be chosen so that pulse amplifier803 produces an output waveform 805 with duty cycle greater than 50% andequal to the desired duty cycle for the desired operation of outputstage 801. Each pulse amplifier has a load network 806, 807, comprisedof the intrinsic parasitic impedance (represented in FIG. 8 by capacitorCL in series with resistor RL) of the device in the class DE amplifier,and a shunt inductor with a bypass capacitor. The bypass capacitor maypresent the shunt inductor with a virtual ground by virtue of the bypasscapacitor's connection to true ground. However, the invention is notlimited as to the particular topology or arrangement of components ofthe load network. Load networks 806 and 807 can be designed to enablezero-voltage-zero-current switching of the driver stage transistors,resulting in high efficiency for the complete amplifier circuit.

In the embodiments described herein, for a desired output duty cycle,the duty cycles of the input pulses to the pulse amplifier may beselected so as to maximize efficiency. For example, the total OFF time φmay be held constant while the duty cycles of the input signals areadjusted accordingly to maximize efficiency. Under some conditions, anefficiency of greater than 70% may be achieved using the techniquesdescribed herein, however the invention is not limited in this respect,as the efficiency achieved may be higher or lower. In someimplementations the efficiency may be at least 50%, while someimplementations may achieve an efficiency of greater than 80% or even90%. As used herein, the term efficiency refers to the ratio of inputpower to output power.

FIG. 9 shows a circuit 900 that is configured to functiondifferentially, according to another embodiment. This circuit addressesa problem that may arise as a result of the capacitance of the largeblocking capacitors shown in series with the shunt inductors in loadnetworks 404, 504, 604, 806, and 807. The capacitance of thesecapacitors typically must be large enough so that the impedance of thecapacitor is significantly less than that of the inductor, e.g., muchgreater than ten times less. However, such a capacitance may beinconsistent with semiconductor manufacturing processes. This difficultycan be overcome by implementing the full amplifier circuit in adifferential manner. FIG. 9 shows a circuit 900 similar to the 2-stageamplifier circuit of FIG. 8, but which has been split into two halvesand driven 180° out of phase. As shown, the circuit 900 comprises two2-stage class DE amplifiers 901 and 902. Amplifier 901 includes twopulse amplifier drivers 903 and 904, each with separate load network 909and 910, and output class DE stage 905. Amplifier 902 includes two pulseamplifiers 906 and 907, each with separate load networks 911 and 912,and an output class DE stage 908. Input signals 1-4 can be chosen so asto drive pulse amplifiers 903 and 904 consistent with the operationdescribed in FIGS. 5 and 6 respectively. Input signals 5-8 can be chosenso as to be identical to input signals 1-4 respectively, but phaseshifted by 180°. This may result in operation identical to amplifier 901and consistent with a previously described embodiment of the invention,but wherein the generated waveforms are 180° out of phase with thewaveforms generated by amplifier 901. As shown, output waveforms fromamplifiers 901 and 902 are, as a result, 180° out of phase with eachother and can be combined using a balun 915 to produce an output signalwith twice the power of a single two-stage amplifier alone. Loadnetworks 909 and 911 can be coupled together via connection 914. Becausepulse amplifiers 904 and 907 operate 180° out of phase, connection 914becomes a virtual ground. The bypass capacitors shown in the embodimentsof FIGS. 4, 5, and 6 can thereby be eliminated. Likewise, load networks910 and 912 can be coupled via connection 913, creating a virtual groundand eliminating further bypass capacitors.

As discussed above, the techniques of the present application and thosedescribed in a U.S. patent application filed on even date herewithentitled “Distributed Multi-Stage Amplifier” may advantageously be usedin combination with one another for providing efficient amplification.However, these techniques need not be used together and can be utilizedseparately, as the invention is not limited in this respect.

Some of the techniques described herein relate to operating an amplifierin a class DE mode of operation. However, in some circumstances asufficiently high efficiency can be achieved by operating a class Damplifier in a manner that is close to class DE operation but not “true”class DE operation. For example, a relatively small voltage and/orcurrent may be present at the terminals of a transistor upon switching,but the resultant power loss may be acceptably small. Such techniquesare within the scope of this disclosure.

Having thus described several aspects of at least one embodiment of thisinvention, it is to be appreciated various alterations, modifications,and improvements may be made within the spirit and scope of theinvention. Accordingly, the foregoing description and drawings are byway of example only.

1. A method of operating a class D push-pull amplifier to generate apulsed output signal, the class D push-pull amplifier comprising a pushtransistor and a pull transistor, the method comprising: driving thepush transistor with a first input signal having a first duty cycle;driving the pull transistor with a second input signal having a secondduty cycle; and controlling the first and second duty cycles of thefirst and second input signals such that the pulsed output signal has adesired duty cycle.
 2. The method of claim 1, wherein the first andsecond duty cycles are different from one another.
 3. The method ofclaim 2, wherein one of the first and second duty cycles is less than50% and another one of the first and second duty cycles is greater than50%.
 4. The method of claim 1, wherein the first and second duty cyclesare selected such that, for a given duty cycle of the pulsed outputsignal, the efficiency of the class D push-pull amplifier is maximized,wherein the efficiency is the ratio of input power to output power. 5.The method of claim 4, further comprising operating the class Dpush-pull amplifier in a class DE mode of operation.
 6. The method ofclaim 5, wherein the efficiency is at least 70%.
 7. The method of claim1, wherein the first and second input signals are pulse-width-modulatedsignals.
 8. The method of claim 1, wherein the output of the class Dpush-pull amplifier comprises a square or trapezoidal waveform.
 9. Themethod of claim 1, wherein controlling the first and second duty cyclescomprises increasing one of the first and second duty cycles anddecreasing a different one of the first and second duty cycles such thata sum of the first and second duty cycles is held at least substantiallyat a constant.
 10. The method of claim 9, wherein the constant is lessthan one.
 11. A circuit for amplifying a signal, the circuit comprising:a class D amplifier; and at least one input circuit that provides atleast two pulsed input signals to the class D amplifier to drive theclass D amplifier, the at least two pulsed input signals havingdifferent duty cycles.
 12. The circuit of claim 11, wherein the class Damplifier comprises a push-pull amplifier comprising a push transistorand a pull transistor, and wherein the at least two pulsed input signalscomprises: a first pulsed input signal having a first duty cycle; and asecond pulsed input signal having a second duty cycle; wherein the firstpulsed input signal drives the push transistor and the second pulsedinput signal drives the pull transistor.
 13. The circuit of claim 12,wherein the first and second duty cycles are selected such that theefficiency of the circuit is maximized, wherein efficiency is the ratioof input power to output power.
 14. The circuit of claim 11, wherein theclass D amplifier is operated in a class DE mode.
 15. The circuit ofclaim 14, wherein the circuit operates with an efficiency of at least70%, wherein efficiency is the ratio of input power to output power. 16.The circuit of claim 12, wherein the first and second duty cycles areselected such that the output signal has a desired duty cycle.
 17. Thecircuit of claim 16, wherein the output signal is a pulsed signal. 18.The circuit of claim 17, wherein the pulsed signal comprises a square ortrapezoidal waveform.
 19. The circuit of claim 11, wherein the class Damplifier generates a substantially unfiltered output that drivesanother amplifier.
 20. The circuit of claim 11, wherein the at least oneinput circuit comprises at least one pulse-width-modulator.